1. Field of the Invention
The present invention relates to a method for manufacturing a half-metallic magnetic oxide, and more particularly to a method for manufacturing a half-metallic magnetic oxide such as Fe3O4 by forming a magnetic field on a substrate and supplying additional RF (Radio Frequency) power thereto and a plasma sputtering reactor used in the method.
2. Description of the Related Art
Generally, half-metallic ferromagnetic oxides have an energy gap to a minority spin band but do not have an energy gap to a majority spin band in Fermi energy level. The formation of the spin-dependent energy gap causes the ferromagnetic oxides to have 100% spin-polarization. These half-metallic substances have very large reluctance in a device having a spin-tunneling structure.
Therefore, since the half-metallic ferromagnetic oxides are used as magnetic recording media, and much research on half-metallic ferromagnetic oxides has been recently carried out. Particularly, the half-metallic ferromagnetic oxides are spotlighted as substances applied to various high-speed and high frequency devices such as magnetic random access memory (MRAM) devices, magnetic sensors, next generation spin devices, etc.
Fe3O4, as a representative half-metallic ferromagnetic oxide, has an inverse spinel structure. The inverse spinel structure is obtained by adding Fe3+ ions on a tetrahedral coordinate and adding Fe2+ and Fe3+ ions on an octahedral coordinate on a face center cubic structure made of oxygen. Specifically, the composition of Fe ions is precisely in the ratio of Fe3+:Fe2+=2:1. Since minority spin electrons hop between Fe3+ and Fe2+ ions, Fe3O4 is excellent in conductivity. The hopping minority spin electrons represent a metal-insulator transition due to a hopping electron frozen effect at Verwey temperature of approximately 125K. Therefore, Fe3O4 has state density in Fermi level at room temperature, even if the state density is very low.
In order to manufacture a thin film made of Fe3O4 being one of the half-metallic magnetic oxides, Fe2+ and Fe3+ ions having different electrovalences must be generated from Fe and the ratio of Fe2+ and Fe3+ ions must be precisely controlled. Further, in order to carry out the above-described deposition process, high power is required to decompose oxygen molecules.
Therefore, the same substance must be decomposed into ions with different electrovalences in the precise ratio in the method for manufacturing the half-metallic magnetic oxide, and high power to decompose the substance is required. As a result, the conventional method for manufacturing the half-metallic magnetic oxide cannot be practically used.
Conventionally, a molecular beam epitaxy (MBE) apparatus and a pulsed laser deposition (PLD) apparatus are used to manufacture half-metallic magnetic oxides. The MBE apparatus has the advantage of precisely controlling the composition ratio. On the other hand, the PLD apparatus has the advantage of using high power in depositing a thin film, thereby being suitably used in a method for manufacturing Fe oxides. Although the MBE process using the MBE apparatus achieves a precise composition ratio, since the MBE apparatus is very expensive, it is difficult to practically use the MBE process. On the other hand, the PLD process using the PLD apparatus cannot use a Fe target, but uses a high-priced Fe3O4 target comprising ions with different equivalences in the desired composition ratio. Further, a magnetic body grown by the PLD process has an antiferromagnetic phase formed therein, thereby not obtaining magnetic saturation.
In the aforementioned conventional processes, magnetic properties of the grown magnetic body may be depreciated by heat energy additionally applied in order to raise the temperature of a substrate. This causes a serious problem of loss of magnetic properties of other magnetic materials, when a multi-layered device is manufactured by stacking the Fe3O4 thin film and other magnetic substances.
As described above, in order to manufacture a half-metallic oxide thin film, there is required a high-priced facility. Further, with the conventional processes, it is difficult to grow a magnetic thin film having excellent magnetic properties. Therefore, the aforementioned conventional processes cannot be practically used in industrial applications.
Therefore, a new method for manufacturing a half-metallic oxide such as Fe3O4 having excellent magnetic properties using a conventional thin film formation apparatus is required.
Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a method for manufacturing a half-metallic magnetic oxide using a conventional sputtering apparatus, which improves oxygen decomposition and properly controls the composition ratio of metal ions and oxygen ions, thereby providing a half-metallic magnetic oxide having excellent magnetic properties.
In accordance with one aspect of the present invention, the above and other objects can be accomplished by the provision of a method for manufacturing a half-metallic magnetic oxide thin film on a substrate using a plasma sputtering apparatus provided with a substrate holder for supporting the substrate, a metal target being opposite to the substrate holder, a gas injection inlet for supplying oxygen gas into a reaction chamber, and a power supply unit for applying a voltage to the reaction chamber using the target and the substrate holder as both electrodes, comprising the steps of:
disposing a conductor provided with at least one hole between the substrate holder and the metal target;
forming a magnetic field at an area where a thin film is formed on the substrate, said magnetic field having a coercive force larger than that of the thin film to be formed;
injecting oxygen gas into the reaction chamber;
applying a designated voltage to the reaction chamber so as to form a sputtering condition; and
forming a half-metallic magnetic oxide thin film on the substrate by bonding ions discharged from the metal target to oxygen ions decomposed from the oxygen gas under the sputtering condition of the reaction chamber.
Preferably, the metal target may be made of iron (Fe), and the half-metallic magnetic oxide thin film formed on the substrate may be a Fe3O4 thin film. Further, the conductor may be ring-shaped. Alternatively, the conductor may be formed as a mesh including a plurality of holes arranged thereon.
Further, preferably, the magnetic field formed on the substrate may have a strength of more than 100 Oe, and more preferably, have a strength between 100 Oe and 2 kOe.
Preferably, the flow rate of oxygen gas injected into the reaction chamber may be controlled in the range of approximately 0.1 sccm to 10 sccm. If necessary, designated activation gas may be injected into the reaction chamber. Preferably, the activation gas may be Argon (Ar) gas.
Preferably, the flow rate of Argon (Ar) gas injected into the reaction chamber may be controlled in the range of approximately 10 sccm to 50 sccm.
Further, preferably, in order to obtain a magnetic thin film having more excellent magnetic properties, a designated RF voltage may be applied to the conductor.
Moreover, preferably, the substrate on which the magnetic thin film is formed may be heated to a designated temperature. In order not to degrade the magnetic properties of the thin film to be formed, the temperature of the heated substrate may be preferably maintained at approximately 100xc2x0 C. to 400xc2x0 C.
In accordance with another aspect of the present invention, there is provided a plasma sputtering apparatus comprising: a reaction chamber; a substrate holder formed on a side of the reaction chamber and serving to dispose a substrate thereon; a gas injection inlet for supplying reaction gas into the reaction chamber; a target being opposite to the substrate and serving to discharge its particles by the designated sputtering in the reaction chamber; a power supply unit for supplying power for generating discharge between the substrate holder and the target; a conductor disposed between the target and the substrate holder and provided with at least one hole for allowing the particles discharged from the target to proceed to the substrate; and magnetic formation means for forming a magnetic field on the substrate disposed on the substrate holder, the magnetic field having a coercive force larger than that of a thin film to be formed on the substrate.